Provide the _subscribed_ topics to the protocol() function

The `topics` field may change depending on the actually assigned topics, and we might effectively
lose topics here when the assigner tries to build a full view of the topics used by the members.

See also https://kafkajs.slack.com/archives/CF6RFPF6K/p1572986319131500

src/consumer/consumerGroup.js

@@ -95,7 +95,11 @@ module.exports = class ConsumerGroup {
       sessionTimeout,
       rebalanceTimeout,
       memberId: this.memberId || '',
-      groupProtocols: this.assigners.map(assigner => assigner.protocol({ topics: this.topics })),
+      groupProtocols: this.assigners.map(assigner =>
+        assigner.protocol({
+          topics: this.topicsSubscribed,
+        })
+      ),
     })
 
     this.generationId = groupData.generationId